In monitoring the blood pressure at a given point in the blood vessels of a patient, a catheter may be inserted through the blood vessels until one end reaches the point of interest. A slow flow of saline solution is maintained through the catheter in order to prevent it from becoming clogged by the coagulation of blood. In order to wash blood from the catheter after a blood sample is drawn through it or to shorten the time for filling the dome that couples the blood pressure to a transducer, means are provided for causing a momentary fast flow of saline solution through the catheter. As the amount of saline solution that can be safely injected into the blood stream of a patient is limited, the means must be designed so as the cut off the fast flow within a short time after the measure is deactivated. It is important that the means be free from pockets in which bubbles can gather that would deteriorate the blood pressure signals derived and that inpurities be prevented from reaching the catheter.